The present invention relates to the measurement of the internal dimensions of an elongated borehole, such as within an oil well.
Apparatus are known for measuring the inner diameters of cylindrical tubes or of wells bored in the ground. For example, inside calipers or sondes capable of being moved in these tubes or boreholes can be equipped with fingers adapted to spread away from the body of the caliper or sonde until they come into contact with the walls of the tubes or boreholes, the measurement of this outward spread of the fingers thus furnishes information relative to the sought diameter.
Such apparatus are also used in uncased boreholes to check the condition of their walls and notably to detect the presence of caving irregularities which can affect the logging measurements to be made within the boreholes. The condition of the internal wall of a tube, such as a steel casing supporting the walls of a well, or a production tubing designed to bring to the surface the fluids produced by a given zone of the earth formations traversed by the well can also be checked. These casing and tubing undergo numerous abrasion and corrosion phenomena with time. The monitoring of their internal dimensions makes it possible to check their degree of wear.
Mechanical caliper apparatus are however relatively complex and difficult to design, especially when one wishes to obtain a large number of measurements in holes of small diameter. Mechanical calipers also have the drawback of probably scratching the wall of the steel tubes into which they are introduced.
Acoustic-type caliper devices are known in which a transducer mounted on the sonde transmits pulses in the direction of the borehole wall. These pulses are reflected by this wall with the resulting echo being detected either by the transducer producing the pulses or by another transducer specialized in the reception of these signals. The time elapsing between the transmission of each pulse and the detection of the corresponding echo provides a measurement of the distance of the transducer from the borehole wall. By repeating similar measurements around the longitudinal axis of the sonde, for example by having a transmitting-receiving transducer rotate around this axis, it is possible to obtain plotting of the form of the hole, the accuracy of which is dependent on the number of measurements made during any given rotation cycle. By sequentially moving the sonde longitudinally after a rotation cycle, it is possible to obtain an image of the form of the hole over any chosen depth interval. Such apparatus however require the use of a drive device for rotating a relatively complex transducer. They must also operate within severe environments such as those encountered in oil well boreholes in which extremely high temperature and pressure conditions often prevail and in which any contacting media can be very abrasive.
Devices have been proposed for measuring one transverse dimension of a borehole wall in a well by means of a transducer mounted in a stationary manner on the sonde. Acoustic transducers designed for this type of application are relatively voluminous. They generally include a piezoelectric disc, one face of which is designed to transmit and receive acoustic signals, the other faces being lined with an absorbant material in order to attenuate the effect of the echoes reflected by the borehole wall in directions other than that of the transmitting face. Owing to their dimensions, it is difficult to consider the use of a large number of such transducers on the same tool, for exploring the dimensions of the borehole in several directions around the axis of the well. In addition, because of their volume, these transducers cannot be used in sondes intended to go through production tubing which is typically of a smaller diameter.